PREPARATORY PROBLEM 30 (PRACTICAL) Analysis of fire retardants by potentiometric titration The purpo — Physical Chemistry — Kinetics Chemistry Question
Analysis of fire retardants by potentiometric titration
PREPARATORY PROBLEM 30 (PRACTICAL)
Analysis of fire retardants by potentiometric titration
The purpose of the experiment is to determine the composition of a mixture simulating a fire retardant containing (NH4)2HPO4 and NH4Cl. First, the sample is dissolved in HCl and titrated with NaOH to determine the amount of phosphoric acid, the best precision being achieved if potentiometric titration (pH values recorded with a pH meter) is used. Generally, titration of a mixture of hydrochloric and phosphoric acids with an alkali results in two end points (inflexions in the titration curve). The first end point indicates the total amount of hydrochloric and phosphoric acids, while the second one corresponds to the completion of the second stage neutralization of phosphoric acid. In this experiment, the second end point cannot be observed due to the formation of ammonium buffer.
To determine the concentration of the ammonium salt, the formaldehyde method is used. The reaction between formaldehyde and ammonium produces the hexamethylene tetrammonium cation (CH2)6(NH+)4, which is more acidic than the NH4 + cation. Another potentiometric titration is necessary to find the total amount of (CH2)6(NH+)4, and thus calculate the total amount of diammonium phosphate and ammonium chloride in the sample.
The acidity constants of phosphoric acid:
Ka1 = 7.1 ⋅ 10–3, Ka2 = 6.2 ⋅ 10–8, Ka3 = 5.0 ⋅ 10–13.
Chemicals and Reagents
* Mixture of (NH4)2HPO4 and NH4Cl, about 1 : 1 by mass
* Sodium hydroxide, solution, c(NaOH) = 0.1 mol dm-3
* Hydrochloric acid, c(HCl) = 0.1 mol dm-3
* Formaldehyde, 20 % CH2O (aq)
Equipment and Glassware
* Analytical balance (± 0.0001 g)
* Volumetric pipette, 10 cm3
* Pipette pump
* Burette, 25 cm3
* Beaker, 100 cm3
* Volumetric flask, 100 cm3
* Magnetic stirrer
* Stirring bar
* pH meter
Procedure
A. Determination of phosphate amount as phosphoric acid
a) Weigh about 0.6 g of the test mixture and place it in a 100 cm3 volumetric flask. Fill with water up to the mark.
b) Transfer 10 cm3 of the prepared solution into a 100 cm3 beaker using a 10 cm3 volumetric pipette. Add 10 cm3 of hydrochloric acid (0.1 mol dm-3, concentration known exactly) using a 10 cm3 volumetric pipette, and dilute it with 20 cm3 of distilled water. Place the beaker onto a magnetic stirrer and put in the stirring bar.
c) Titrate the sample with a solution of sodium hydroxide (0.1 mol dm-3) adding it by 0.5 cm3 portions until the pH starts increasing. Continue adding the titrant in drop portions. When the change of pH with each added portion significantly decreases, continue titration with larger portions of sodium hydroxide. Record the volume of sodium hydroxide added and each pH value measured.
d) Repeat the titration with new aliquots of the sample solution as needed to obtain consistent results.
B. Determination of the total amount of ammonium salts
e) Prepare a 20% aqueous solution of formaldehyde free of formic acid. Neutralize the solution with sodium hydroxide, if needed. Use titration in the presence of phenolphthalein to determine the necessary amount of NaOH for the neutralization.
f) Transfer 10 cm3 of the sample solution into a 100 cm3 beaker using a 10 cm3 volumetric pipette. Add 5 cm3 of the formaldehyde solution and wait for 2 min.
g) Place the beaker onto the magnetic stirrer and put in the stirring bar. Titrate the sample with a solution of sodium hydroxide (0.1 mol dm-3) with constant stirring as described in part A.
h) Repeat the titration with new aliquots of the sample solution as needed to obtain consistent results.
How many end points are expected during the titration of a mixture of H3PO4 and HCl?
Model Answer
Titration curves for a polyprotic acid (such as phosphoric acid) or a mixture of acids are characterized by more than one endpoint if Ka1 : Ka2 >= 10^4 and the equilibrium constant of acidity of the weak acid is more than n * 10^-9. The equilibrium constants of acidity of phosphoric acid are: Ka1 = 7.1 * 10^-3, Ka2 = 6.2 * 10^-8, Ka3 = 5.0 * 10^-13. Thus, there are two breaks on the titration curve of phosphoric acid (Fig. 1). The third break is not observed due to very low value of Ka3.
[VISUAL]
During titration of a mixture of hydrochloric and phosphoric acids, the proton of hydrochloric acid and the first proton of phosphoric acid react with sodium hydroxide simultaneously. By the second endpoint H2PO4- is converted into HPO4 2-.
Can color indicators be used in the determination of concentrations of hydrochloric and phosphoric acids in their mixture?
Model Answer
The first and second equivalence points of H3PO4 are observed at pH of about 4.7 and 9.6, respectively. For determination of hydrochloric and phosphoric acids in their mixture, one can use indicators with color change around these pH values (for example, bromocresol green and thymol phthalein for the first and second titrations, respectively).
Write down the equations of all the reactions occurred.
Model Answer
The following reaction takes place on addition of HCl to the sample:
(NH4)2HPO4 + 2 HCl → 2 NH4Cl + H3PO4
Formaldehyde reacts with ammonium salts to form hexamethylenetetrammonium cation:
4 NH4+ + 6 H2CO → (CH2)6(NH+)4 + 6 H2O
The equations describing the titration of hexamethylenetetrammonium salt, hydrochloric and phosphoric acids with sodium hydroxide:
(CH2)6(NH+)4 + 4 OH- → (CH2)6N4 + 4 H2O
HCl + NaOH → NaCl + H2O
H3PO4 + NaOH → NaH2PO4 + H2O
NaH2PO4 + NaOH → Na2HPO4 + H2O
Plot the graphs of pH, dpH / dV, and d^2pH / dV^2 vs. volume of the titrant added. Find the end points from the curves analysis. Why is there only one end point in the titration curve of hydrochloric and phosphoric acids in the presence of ammonium ion?
Model Answer
A typical analysis of potentiometric titration data is shown in Fig. 2. The most steeply rising portion on the curve (a) corresponds to the endpoint, which can be found more precisely by studying dependences of the first (maximum on curve (b)) or second (zero value on curve (c)) derivatives. In the presence of ammonium salts, the reaction corresponding to the second end point in H3PO4 titration H2PO4- + OH- → HPO4 2- + H2O is overlaid by the process NH4+ + OH- → NH3 + H2O, which makes the potential rise gradually rather than sharply (ammonium buffer).
[VISUAL]
Fig. 2. Typical plots of potentiometric titration: (a) titration curve of an acid with a base; (b) curve of the first derivative; (c) curve of the second derivative.
Calculate the content (in weight %) of (a) diammonium phosphate and (b) ammonium chloride in the test sample.
Model Answer
(a) Calculation of phosphate amount
With VNaOH,1 designating the volume of sodium hydroxide used in titration A, the amount needed to neutralize hydrochloric acid and the first proton of phosphoric acid is:
nPO4 + nHCl (titrated) = cNaOH * VNaOH,1
At the same time,
cHCl * VHCl (added) = nHCl (titrated) + 2 nPO4 (HCl spent for the reaction with (NH4)2HPO4))
Then,
nPO4 = cHCl * VHCl (added) - cNaOH * VNaOH,1
Since n(NH4)2HPO4 = nPO4, one finally gets:
w(NH4)2HPO4 = (10 * nPO4 * M(NH4)2HPO4 ) / mmixture
(b) Calculation of the total amount of diammonium hydrophosphate and ammonium chloride
With VNaOH,2 designating the volume of sodium hydroxide used in titration B (that is, spent for the neutralization of hexamethylene tetrammonium cation (CH2)6(NH+)4 obtained from the ammonium salts), one gets:
nNH4Cl + 2 nPO4 = cNaOH * VNaOH,2
The amount of phosphate n(NH4)2H3PO4 was determined in experiment A, which allows calculating the amount of NH4Cl
nNH4Cl = cNaOH * VNaOH,2 - 2 (cHCl * VHCl - cNaOH * VNaOH,1)
and its content in the mixture:
wNH4Cl = (10 * nNH4Cl * MNH4Cl ) / mmixture